The present invention relates to a device for measuring the oxygen saturation of arterial blood. More particularly, the present invention relates to an improved non-invasive oximeter and method for mathematically processing the oxygen saturation calculation independent of pulse determination.
Oximetry is the determination of the oxygenation level of the blood. One constituent of human blood is hemoglobin. Hemoglobin which is contained in red blood cells, picks up oxygen from the lungs and carries the oxygen to the body cells. Blood traveling from the lungs to the body cells with oxygen is called arterial blood. Blood traveling to the lungs from the body cells with diminished oxygen is called venous blood. Oximeters function by measuring the oxygen saturation, the amount of oxygenated hemoglobin as a percentage of total hemoglobin, in arterial blood.
The blood oxygen saturation of a patient is one indication of a patient's pulmonary health. In the operating room, blood oxygen saturation is an indication of whether an anesthetized patient is receiving sufficient oxygen. A low oxygen saturation measurement is a warning of dangerous oxygen deprivation, or hypoxemia, a potential cause of injury or death.
Prior to the development of non-invasive oximeters, the oxygen saturation of blood was determined "in vitro", commonly in a container called a cuvette Measurements are first made of the light transmitted through a cuvette filled with a saline solution. This provides a "bloodless" reference measurement for use in the oxygen saturation calculation The cuvette is then filled with blood and a second set of measurements of transmitted light intensity is taken, to provide "blood-filled" measurements at two wavelengths The foregoing measurements of light intensity are converted to absorption values and are then used with standard equations to solve for blood oxygen saturation
Once non-invasive oximeters were developed, the necessity of taking blood from the patient was avoided Non-invasive oximeters are now well known and are used widely to measure oxygen saturation Oximeters function by passing light of various colors or wavelengths through a sample. On the human body, typical measuring points are the tip of a finger or an ear lobe. The sample absorbs the transmitted light to varying degrees relative to the particular constituents through which the light passes A photosensitive device, such as a photo multiplier tube or photodiode, is used to detect the transmitted light Alternatively, the photosensitive device can be designed to detect the light reflected from the sample. Either system provides a measure of the light the sample absorbs, i.e., the light the sample does not transmit or reflect Using measurements of the transmitted light intensity, the absorption of light by the sample can be calculated. Calculations can then be made of the percentage of the particular constituent of interest in the sample.
In general, methods for measuring oxygen saturation utilize the relative difference between the light absorption (or attenuation) coefficient of oxygenated hemoglobin and that of reduced hemoglobin. The light absorption coefficient for oxygenated hemoglobin and reduced hemoglobin is dependent on the wavelength of the light traveling through them. Both oxygenated hemoglobin and reduced hemoglobin transmit light having a wavelength in the infrared region to approximately the same degree However, in the visible region, the light absorption coefficient for oxygenated hemoglobin is quite different from the light absorption coefficient of reduced hemoglobin. The two colors typically chosen to shine through the blood sample are red light and infrared light. In oximeters, light intensity is measured at various physiological states The beating of the heart provides the various states. As the heart beats, arterial blood is forced in the arteries and capillaries to produce a blood filled state. The blood then drains leaving a reference which consists of tissue, bone and some amount of venous blood. The collected transmitted light is subjected to photoelectric conversion and then mathematical conversion to eventually calculate the degree of oxygen saturation in the blood.